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Author Topic: A Model Dirigible  (Read 4039 times)
dantes_torment
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« on: March 10, 2008, 06:03:04 pm »

I want to make a small version of a dirigible(perhaps a Mindenburg?) out of aluminum flashing. I hope to make a rigid shell which I will then evacuate to the best of my ability, fill with hydrogen, then evacuate again. That way, I will end up with a near vacuum, with the only molecules inside being hydrogen, which will be notably lighter than merely hydrogen filled, and notably more plausible than vacuum "filled".

Thoughts?
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Smaggers
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« Reply #1 on: March 10, 2008, 06:36:23 pm »

I would think that in order to be strong enough to resist the effects of air pressure it would end up too heavy.
I'm obviously quite happy to be proved wrong though.
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dantes_torment
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« Reply #2 on: March 10, 2008, 06:39:49 pm »

It would be a cross between a vacuum and hydrogen filled, i.e. low pressure hydrogen. I'm trying to find the right pressure, at which the shell is light enough to be lifted by the gas inside.
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cuchlann
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« Reply #3 on: March 10, 2008, 06:43:11 pm »

This sounds very interesting.  I'll be interested to see how it goes -- in fact, I might like to try it myself, but I'm faily build-naive.  What is "aluminum flashing?"  Please let us know how you get on.
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akumabito
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« Reply #4 on: March 10, 2008, 06:43:44 pm »

I want to make a small version of a dirigible(perhaps a Mindenburg?) out of aluminum flashing. I hope to make a rigid shell which I will then evacuate to the best of my ability, fill with hydrogen, then evacuate again. That way, I will end up with a near vacuum, with the only molecules inside being hydrogen, which will be notably lighter than merely hydrogen filled, and notably more plausible than vacuum "filled".

Thoughts?

Why go through all the trouble? Hydrogen is quite a bit lighter than air, ergo, if you fill the dirigible from the bottom, the hydrogen will collect at the highest point, forcing the normal air down. Make sure the air can escape though an opening near the fill point, and wait until you have filled the dirigible until hydrogen starts coming out of the air gap. Presto, dirigible filled with 100% pure hydrogen. And if I am not mistaken, this is pretty much the way all lighter-than-air craft are filled.. Wink
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Ben Hudson, Esq.
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« Reply #5 on: March 10, 2008, 06:51:30 pm »

I would have to say that with my limited experience (ie., none), it would probably be easier to go with Mr Akumabito's suggestion. A partial vacuum would be better, but would need more structural strength. How big are you considering making this dirigible?
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dantes_torment
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« Reply #6 on: March 10, 2008, 06:57:21 pm »

Cigar shaped; I'll make maybe 12 or so long, thin strips of aluminum flashing which taper at either end. I'll weld/solder at the seams, and put a valve at one end.
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polyphemus
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« Reply #7 on: March 10, 2008, 07:10:19 pm »

I seem to remember that atmospheric pressure is 14 lbs per sq in or so. Getting flashing to resist that without collapse even at partial vacuum is going to be a challenge...
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dantes_torment
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« Reply #8 on: March 10, 2008, 07:15:57 pm »

I might put some supports inside(think bird bones).
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Ben Hudson, Esq.
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« Reply #9 on: March 10, 2008, 07:20:47 pm »

I had thought about that. What about aluminium boning inside, with some sort of impermeable and unstretchy fabric on the outside?
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dantes_torment
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« Reply #10 on: March 10, 2008, 07:27:46 pm »

I had thought about that. What about aluminium boning inside, with some sort of impermeable and unstretchy fabric on the outside?

Thought about it, didn't seem sturdy enough.
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MPsy
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« Reply #11 on: March 10, 2008, 07:47:28 pm »

Oooooo...I hope you post up pictures when you're done.
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akumabito
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« Reply #12 on: March 10, 2008, 07:50:46 pm »

Work out the volume of the dirigible first. The calculate max lift assuming 100% hydrogen.
Then work out how heavy the aluminum siding is going to be.

Just to make sure the thing will actually fly you see.. Smiley
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dantes_torment
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« Reply #13 on: March 10, 2008, 08:16:35 pm »

Of course.
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Mr. Consciousflesh
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« Reply #14 on: March 10, 2008, 08:32:34 pm »

There is a simpler and safer method of filling the dirigible with hydrogen - fill it first with inert gas at atmospheric pressure. Argon and nitrogen will be easiest to get since these gases are commonly used for welding. This way You can avoid creating explosive mixture or crushing model's hull with too much pressure difference .
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« Reply #15 on: March 10, 2008, 08:54:39 pm »

The problem is that neither of those have the same lifting power as hydrogen.
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Mr. Consciousflesh
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« Reply #16 on: March 10, 2008, 09:04:34 pm »

I think You didn't understand me. First You have to fill the dirigible with inert gas and purge all remaining air with it and then fill it with hydrogen. This way at no time the explosive oxygen-hydrogen mixture is made and the pressure during the whole procedure is constant. In the end You will get dirigible filled with hydrogen with some inert impurities.
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Doctor Trakov
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« Reply #17 on: March 10, 2008, 10:09:51 pm »

Ah, I see what you mean.
The initial cost would be slightly higher, perhaps, but it's worth not exploding.
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Prof. Brockworth
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« Reply #18 on: March 10, 2008, 10:42:16 pm »

Pfff, you fear-of-exploding pantywaists will be the death of this Empire!

Alu flashing is interesting though.  Pretty sturdy stuff (I have a couple of spare rolls after sending someone else shopping for gaffer tape, never again).  And (says he cutting a piece for testing) a 5cm x 10cm piece weighs under a gramme.

Here are the lift tables you need!

http://www.chem.hawaii.edu/uham/lift.html

All you need to do is work out your envelope area...

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Dr cornelius quack
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« Reply #19 on: March 10, 2008, 10:47:16 pm »

Good experiment to try is to part fill an aluminium beer can with water, put it on a hot plate to boil, when it's boiling nicely i.e. lots of steam, take some tongs and invert the can into a washing up bowl of cold water so that the spout is submerged. Its quite drammatic.

In the meantime can any one tell me how to get my flask of coffee down from the ceiling?

Yours, thirstily

Dr. Q.
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Prometheus Culier
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« Reply #20 on: March 10, 2008, 10:57:44 pm »

what about a very lightweight bag within the aluminum shell?  You evacuate the bag, which collapses, then fill it with hydrogen.  As it fills it expands and displaces the air within the aluminum shell.  That solves your problem of having to build structural integrity to withstand vacuum.  You also don't have to worry about a perfect solder job to seal the shell.

I am under the impression that full scale rigid dirigibles contained their hydrogen in cells very much like this.

For what it is worth, your model is unlikely to generate sufficient lift to launch itself. Think about mylar balloons of a similar size and how much lift they generate. Something as small as a clothespin is usually enough to hold them down; I expect your aluminum shell will weigh at least that much.  They use helium, not hydrogen, but the principle remains.   If you want it to fly you will have better luck with a blimp than a zeppelin.
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Prometheus Culier
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« Reply #21 on: March 10, 2008, 11:03:13 pm »

this may be of interest

http://scientificsonline.com/product.asp_Q_pn_E_3083049
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